In accordance with prior art, the common circuit of flyback power supply device is shown in FIG. 1A. Therein, the component S1 can be a transistor, thyristor, metal-oxide-semiconductor field-effect transistor (MOSFET) or other components that can be switched on or off by a small signal. Generally, there is a voltage drop of 0.4-1.5V across the component D1 with forward bias (a feature of p-n junction diode). Hence, the flyback power supply device will be inefficient while the output voltage V0 is low, or it will need a heat sink with large area due to its large power consumption. For example, if V0 is 5Vdc, the voltage drop across D1 is 0.4V, the voltage limit of reverse bias across D1 is 30Vdc and the output power of the flyback power supply device is 50 W (5V/10 A), the power consumed by D1 will be 0.4V*10 A=4 W. If the power consumed by other components is ignored, the efficiency of the flyback power supply device will be 50 W/(50 W+4 W)=92.6%.
The present circuit of flyback power supply device is shown in FIG. 1B. The component D1 is replaced by the component S2, which can be a transistor, thyristor, or MOSFET. By using the present technology, the on-resistance RDS(on) of MOSFET can reach about 10 mΩ easily, e.g. S14410. Hence, the power consumption can be reduced tremendously to overcome the drawback described above. Comparing with the example above, if V0 is 5Vdc, the S2 is replaced by S14410 (RDS(on)=11 mΩ, VDS=30V) and the output power of the flyback power supply device is 50 W (5V/10 A), then the voltage drop of S2 will be 10 A*11 mΩ=110 m Vdc and the power consumed by S1 will be 110 m V*10 A=1100 m W=1 W. If the power consumed by other components is ignored, the efficiency of the flyback power supply device will be 50 W/(50 W+1.1 W)=97.8%. Hence, comparing with the one using p-n junction diode, there is 6.2% efficient improvement. This is the present aim for engineers to pursue. However, there is still a technical bottleneck while replacing D1 with S1.
The voltage and current waveforms of the conventional flyback power supply device are shown in FIG. 2. The component S1 should be switched on exactly after t1 and off before t2. Usually, t1a is easy to control because t1 refers to the time point that VN2 is changed from negative to positive and hence VN2 can be used as a trigger signal to make S2 being switched on. However, since t2 is changed with the load, it is very hard to forecast. The component S2 should be switched off at t2a before t1. Otherwise, the capacitor Co will charge the inductor N2 via S2 and a reverse current (−IS1) will be produced to burn S1 down while the S1 is switched on again. Although ID1 surely can be used to forecast t2, it is impractical due to the following reasons:                (1) The resistor and current transformer are usually used to detect electric current. However, the resistor will consume the electric power and the power consumption will increase while the electric current increases. It may make the effect of replacing D1 with S1 vanish. Furthermore, the current transformer will remove the effect of direct current. Hence, some circuits should be added to recover the level of direct current and that will reduce the accuracy of current detection significantly.        (2) The accuracy of current detection should be very high. Otherwise, the flyback power supply device may be burned down or have low efficient improvement.        
Therefore, due to the two reasons above, using S2 to replace D1 becomes a expensive and unstable method.
Accordingly, as discussed above, the conventional flyback power supply device still has some drawbacks that could be improved. The present invention aims to resolve the drawbacks in the prior art.